Technical Abstract: Filamentous particles c. 750 nm long and 11 nm in diameter were observed by electron microscopy in leaves of Poa pratensis with mild mosaic symptoms. The virus was easily transmissible by mechanical inoculation ot oat, maize (SDP2) and P. pratensis but not to wheat, rye or barley. The symptoms on oat appeared at first as chlorotic lines on emerging leaves and develops later in irregularly shaped necrotic areas. Based on immunological methods, electron microscopy and host range studies the virus isolated from P. pratensis was identified as an isolate of Oat necrotic mottle virus (ONMV-Pp). The virus did not react with antisera to Agropyron mosaic, Brome streak mosaic (BrSMV), Hordeum mosaic, Maize dwarf mosaic, Ryegrass mosaic, Spartina mottle or Sugarcane mosaic viruses. However, ONMV-Pp was found to have a close serological relationship to the type strain of ONMV deposited in the ATCC as No. PV107. ONMV was also found to be serologically related to several isolates of Wheat streak mosaic virus (WSMV) in DAS-ELISA and in Western blots (CP molecular weight of ONMV-Pp app. 45 kD). The virust was detected in oat plants by IC-RT-PCR using the potyvirus universal primers RCF1 and poty U. The ONMV-Pp PCR product was cloned and sequenced. Sequence comparisons indicated that ONMV-Pp shared between 83.8 and 84.6% identity to different WSMV isolates, but only 56.4% amino acid identity to BrSMV. On the basis of its close relationship to MSMV, it was concluded that ONMV is a member of the genus Tritimovirus and not the genus Rymovirus. ONMV, previously only known from Canada, was identified for the first time in Germany.